Method of electrolytically reducing aromatic hydrocarbons

ABSTRACT

THIS INVENTION RELATES TO A METHOD OF THE PRODUCTION OF DIHYDROAROMATIC HYDROCARBONS WHICH COMPRISES SUBJECTING A CATHOLYTE COMPOSED OF A HETEROGENEOUS MIXTURE OF AN AROMATIC HYDROCARBON AND AN EQUEOUS SOLUTION OF ONE OR MORE QUATERNARY AMMONIUM SALTS TO ELECTROLYSIS CARRIED OUT AT A TEMPERATURE FROM 30* TO 100*C. AND SUBSEQUENTLY RECOVERING THE RESULTING DIHYDRO AROMATIC HYDROCARBON.

United States Patent 3,700,572 METHOD OF ELECTROLYTICALLY REDUCING AROMATIC HYDROCARBONS Takefumi Hatayama, Yoshihiko Hamano, and Tsuyoshi Yamamoto, Nobeoka, Japan, assignors to Asahi Kasei Kogyo Kabushiki Kaisha, Osaka, Japan N Drawing. Filed Dec. 16, 1970, Ser. No. 98,931

Claims priority, application Japan, Dec. 22, 1969, 44/102,440 Int. Cl. C07b 29/06; C07c /10, 13/48 US. Cl. 204-73 R 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of the production of dihydroaromatic hydrocarbons which comprises subjecting a catholyte composed of a heterogeneous mixture of an aromatic hydrocarbon and an aqueous solution of one or more quaternary ammonium salts to electrolysis carried out at a temperature from 30 to 100 C. and subsequently recovering the resulting dihydro aromatic hydrocarbon.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a method of electrolytically reducing aromatic hydrocarbons to produce dihydro aromatic hydrocarbons.

(2) Description of the prior art Electrolytic reduction of aromatic hydrocarbons has been carried out either by electrolysis of a non-aqueous solution using as a solvent ethylamine, ethylenediamine or the like such as those described in Journal of the American Chemical Society 86, 52.72 (1964) and J. Electrochem. Soc. 113, 1060 (1966) or by a method in which the electrolysis is carried out in an electrolytic bath using as a solvent an aprotic solvent such as tetrahydrofuran or diglyme containing a minor amount of water such as one described in J. Electrochem. Soc. 115, 266 (1968). It is also known that, in the electrolytic reduction of tetrahydronaphthalene which is recognized to be as hardly reduced as benzene, a non-aqueous electrolytic bath is employed (of. Journal of the American Chemical Society 89, 186 (1967)).

The prior art processes using organic solvents as mentioned above, however, have great disadvantages for practical use owing to 1) a tendency to induce loss of an expensive solvent, (2) complicated procedures for recovering the resulting dihydro aromatic hydrocarbons and (3) remarkably low electric conductivity of the electrolytic bath as compared with the case of an aqueous solution. For example, in order to recover the dihydro aromatic hydrocarbon efiiciently from the electrolyte using an organic solvent there are needed complicated procedures involving usually distillation of the solvent employed, washing of the residue with water to remove the supporting salt and fractional distillation of the washed residue or fractional distillation of the entire electrolyte. The recovery step as mentioned above, which involves increased number of the components or increased quantity of the liquid to be treated, requires either complicated procedures or wasteful energies and facilities.

Despite these disadvantages the electrolysis using a pure or a high-concentration organic solvent is actually utilized because of the idea as described below which takes possibility of eifecting reduction at a more positive potential due to solvolysis phenomenon into consideration. For example, Whereas benzene requires a potential more 3,700,572 Patented Oct. 24, 1972 ice SUMMARY OF THE INVENTION It is an object of this invention to provide the production of dihydro aromatic hydrocarbons by electrolysis in an organic solvent free heterogeneous mixture. Other objects will be apparent from the descriptions hereinbelow.

After an extensive investigation of the commercial and economic production of dihydro aromatic hydrocarbons, We have now found that production of dihydro aromatic hydrocarbons in a high yield with a high current efficiency can be achieved without use of any organic solvent by carrying out the electrolysis at a temperature from 30 to 100 0, preferably from 50 to C. As a result of the foregoing investigation and discovery we have devised an important method which is advantageous in many respects as mentioned below.

Description of the invention This invention is concerned with a method of the production of dihydro aromatic hydrocarbons formed in a catholyte which is a heterogeneous mixture composed of an aqueous solution of one or more quaternary ammonium salts such as tetra-n-butyl ammonium sulfate and an aromatic hydrocarbon by carrying out the electrolysis under vigorous stirring in an electrolytic bath using mercury as a cathode and having a diaphragm of a material such as an ion exchange membrane. The aromatic hydrocarbon which may be employed in the method of the invention includes mononuclear aromatic hydrocarbons such as benzene or derivatives thereof such as toluene, ethylbenzene, xylenes and diethylbenzenes having alkyl group of carbon number 1-2 and polynuclear aromatic hydrocarbons such as naphthalene or derivatives thereof such as a-methylnaphthalene, fl-methylnaphthalene and a-ethylnaphthalene having alkyl group of carbon number 1-2, the amount of the hydrocarbon employed being preferably from 0.3 to 20 parts by Weight per parts by weight of the aqueous solution of one or more quaternary ammonium salts. Higher amounts of the hydrocarbon will result in lower electric conductivities while on the other hand lower amounts of the same will lower the current efficiency. Especially suitable is the use of an amount from 3 to 10 parts by weight.

As the quaternary ammonium salt suitably used in the method of this invention are mentioned sulfates, halogenides, aliphatic sulfonates, cycloaliphatic sulfonates and the like. Although it is possible to use a quaternary ammonium hydroxide in place of the supporting salt, it is not preferable because of its high corrosivity. However, use of a small amount of a quaternary ammonium hydroxide in combination with the aforementioned quaternary ammonium salts is effective in the provision of stable electrolysis. As the quaternary ammonium group are preferred those consisting of alkyl and cycloalkyl groups, those consisting mainly of n-butyl group such as methyltri-n-butyl ammonium, ethyl-tri-n-butyl ammonium, npropyl-tri-mbutyl ammonium, tetra-n-butyl ammonium and n-amyl-tri-n-butyl ammonium being suitable for practical use with good results. The quaternary ammonium salt is suitably employed at a concentration of several percent by weight or higher, preferably from about to 50% by weight, in view of electric conductivity of the electrolytic bath, and usually from 20 to 30% by weight.

The temperature at which the method of the invention is effectively operated, is a temperature from room temperature up to the boiling point of the catholyte, usually a temperature from 30 to 100 C. being suitable and a temperature from 50 to 80 C. being particularly preferable.

The diaphragm used in this invention is desirably an ion exchange membrane. Materials such as an unglazed pottery plate of fine mesh or a glass filter which substantially prevent admixture between the catholyte and the anolyte and are well conductive when immersed in the electrolytic bath may also be used. As the anolyte is preferably employed sulfuric acid. Other acids such as hydrochloric, hydrobromic, nitric and sulfonic acids may be used, which are preferably used with a cation exchange membrane in view of the control of a electrolyte concentration.

In order to carry out the method of the invention more effectively vigorous stirring of the catholyte is needed. This is satisfactorily made by means of a conventional stirrer or by rapid circulation of the catholyte by a pump. A surface active agent at a concentration of about 0.1% may be added in order to produce a good result of the reduction with moderate stirring applied.

Mercury is most suitable used as the anode with which the method of the invention can be effectively conducted. However, the quality of the cathode material does not directly affect the results of this invention and conventional insoluble materials such as platinum and graphite may be used.

The range of current density at which the method of this invention can be efiiciently carried out is from 0.5 a./dm. to 100 a./dm. and the range between 2 and 50 a./dm. is particularly preferred.

The dihydro aromatic hydrocarbons thus produced, which are sparingly soluble in the aqueous solution of quaternary ammonium salts, are separated for the most part as the liquid upper layer together with the unreacted aromatic hydrocarbon when allowed to stand for a while outside the electrolysis system. The dihydro aromatic hydrocarbons, therefore, can be easily recovered by conventional purifying procedures such as fractional distillation of the upper layer portion. The electrolysis can be continued by recyclizing the separated liquid lower layer in admixture with a fresh starting aromatic hydrocarbon for use as the catholyte.

Because the system is organic solvent free there can be no loss of the solvent at all in the electrolysis step and the step for recovering the dihydro aromatic hydrm rbon according to the present invention, whereas it is impossible or extremely difiicult in the prior methods using an organic solvent to recover the solvent which tends to leak into the anode chamber. The recovery, if any, is usually associated with a considerable loss of the organic solvent.

The electric conductivity of the electrolytic bath used in this invention is very high as exemplified below.

v./cm. 20% aqueous solution of tetra-n-butyl ammonium sulfate at 25 C. 20000 20% aqueous solution of tetra-n-butyl ammonium bromide at 20 C. 18000 As compared with the electric conductivity of the prior art electrolytic bath as exemplified by 0.3 M LiCl-ethylenediamine solution 1860 Diglyme solution containing 20% tetra-n-butyl ammonium bromide and water at 25 C. 4100 0.3 M LiCl solution in 67 molar percent ethanol and 33 molar percent hexamethyl phosphoramide at 25 C. 2500 the values with the electrolyte of the invention are several times higher. In the method of the present invention, therefore, more current is yielded accordingly so that the method is advantageous in that productivity per unit electrolytic cell is largely increased. In addition, the method of the invention can be a continuous process.

The dihydro aromatic compounds are useful as the starting material for polymers and organic chemicals.

PREFERRED EMBODIMENTS OF THE INVENTION To describe this invention more particularly, the following illustrative examples are given.

Example 1.An H-shaped electrolytic cell composed of pure mercury and platinum respectively as the cathode and anode, a strongly acid cation exchange membrane in H form parting the anode chamber and the cathode chamber and a stirrer provided in the cathode chamber was used. As the anolyte was employed 2 N aqueous sulfuric acid and as the catholyte a heterogeneous mixture of tetra-n-butyl ammonium sulfate, tetra-n-butyl ammonium hydroxide, water and benzene in a weight ratio of 20:1:7lz8. While applying vigorous stirring in such a manner that no drop of the benzene was visible and maintaining the temperature at 65 C., electrolysis was run for one hour. Current density on the mercury surface was about 8 a./dm. and current intensity 0.25 F. per mole of the benzene. Gaschromatographic analysis of the catholyte after the electrolysis indicated that formation of cycl0hexa-l,4-diene and cyclohexene alone, the yields of which in terms of current efiiciency were 67% and 10%, respectively, corresponding to a selectivity in the formation of cyclohexa-l,4-diene of 93%.

Example 2.-The same electrolytic cell and anolyte as in Example 1 were used. As the catholyte was employed a 40:5 1:811 heterogeneous mixture of tetra-n-butyl ammonium bromide, Water, toluene and tetra-n-butyl ammonium hydroxide. Electrolysis was made under vigorous stirring while maintaining the temperature of the catholyte at C. Current density on the mercury surface was about 8 a./dm. and current intensity 0.3 F. per mole of the toluene. Gaschromatographic analysis of the catholyte after the electrolysis indicated formation of methylcyclohexa-1,4-diene at a current efficiency of 45%.

Example 3.-The same electrolytic cell and anolyte as in Example 1 were used. As the catholyte was employed a 40:1:51z8 heterogeneous mixture of tetra-n-butyl ammonium bromide, tetra-n-butyl ammonium hydroxide, water and p-xylene was employed. Electrolysis was made under vigorous stirring while maintaining the temperature of the catholyte at 80 C. Current density Was 6 a./dm. and current intensity 0.4 F. per mole of the p-xylene. Analysis of the catholyte after the electrolysis indicated formation of dimethylcyclohexadiene at a current efficiency of 38%.

Example 4.--The same electrolytic cell as in Example 1 was used. As the anolyte was employed 2 N aqueous nitric acid. A 30:l:63:6 heterogeneous mixture of tetran-butyl ammonium bromide, tetra-n-butyl ammonium hydroxide, water and benzene additionally containing 0.1% cetyl-trimethyl ammonium bromide was used as the catholyte. Electrolysis was made at a current density on the cathode surface of 10 a./dm. while maintaining the catholyte at 55 C. under stirring to such a degree as giving an emulsion. Current intensity was 0.25 F. per mole of benzene. Analysis of the catholyte after the electrolysis indicated formation of cyclohexa-l,4-diene at a current efi'iciency of 68% and a selectivity of 92%.

Example 5.-A horizontal electrolytic cell partitioned into the upper and lower chambers by a cation exchange membrane was used in which pure mercury was placed in the lower chamber for use as the cathode and a platinum net in the upper chamber for use as the anode. 0.5 N aqueous sulfuric acid was circulated between the anode chamber and the anolyte storage. In the cathode chamber and the catholyte storage was placed 6 'kg. of a heterogeneous mixture of tetra-n-butyl ammonium bromide and sulfate, water and benzene at a weight ratio of 20: 1:69: 10, respectively, which was vigorously circulated through a heat exchanger for electrolysis while maintaining the temperature at 65 C. Current density on the mercury surface was a./dm. and current intensity 0.4 F. per mole of the benzene. After completion of the electrolysis the catholyte was allowed to stand to separate the upper organic phase from the aqueous phase. The organic phase weighing 580 g. was analyzed to find concentration of cyclohexa-1,4-diene and cyclohexene being respectively 13.2% and 1.0%. Fractional distillation of the organic phase aiforded cyclohexa-1,4-diene at a recovery over 90%.

Example 6-.-The same electrolytic cell and anolyte as in Example 1 were used. As the catholyte Was employed a 40:57z3 by weight heterogeneous mixture of tetra-nbutyl ammonium bromide, water and naphthalene. Electrolysis was made while maintaining the temperature of the catholyte at 80 C. under vigorous stirring. Current density on the mercury surface was about 8 a./dm. and current intensity 0.5 F. per mole of the naphthalene. Gaschromatographic analysis of the catholyte after the electrolysis indicated formation of 1,4-dihydronaphthalene at a current intensity of 72%. Selectivity of the formation of 1,4-dihydronaphthalene was 88%.

Example 7.-The same electrolytic cell and anolyte as in Example 1 were used. As the catholyte was employed a 4225325 by weight mixture of ethyl-tri-n-butyl ammonium sulfate, water and a-methylnaphthalene. Electrolysis was made while maintaining the temperature of the catholyte at 75 C. under vigorous stirring. Current density on the mercury surface was 6 a./d1n. and current intensity 0.8 F. per mole of the ct-methylnaphthalene. Gaschromatographic analysis of the catholyte after the electrolysis indicated formation of 1-methy1-5,8-dihydronaphthalene at a current efliciency of 70% and a selectivity of 89%.

Example 8.The same electrolytic cell and anolyte as in Example 1 were used. As the catholyte was employed a 35:51:14 by weight heterogeneous mixture of n-amyltri-n-butyl ammonium chloride, water and benzene. Electrolysis was made while maintaining the temperature of the catholyte at 67 C. under vigorous stirring. Current density on the mercury surface was 10 a./dm. and current intensity 0.5 F. per mole of the benzene. Gaschromatograp hic analysis of the catholyte after the electrolysis indicated formation of cyclohexa-1,4diene at a current efficiency of and a selectivity of 90.5%.

We claim:

1. Method of the production of dihydro aromatic hydrocarbons which comprises subjecting an organic solvent free catholyte composed of a heterogeneous mixture of an aromatic hydrocarbon to be reduced selected from the group consisting of benzene, naphthalene and alkyl derivatives thereof having 1 to 2 carbon atoms and an aqueous solution of at least one alkyl or cycloalkyl quaternary ammonium salt in which the alkyl or cycloalkyl groups contain from 1 to about 6 carbon atoms to electrolysis carried out at a temperature from 30 to C.

2. Method according to claim 1 wherein the aromatic hydrocarbon is selected from the group consisting of benzene and alkyl derivatives thereof having 1 to 2 carbon atoms.

3. Method according to claim 2 wherein the said quaternary ammonium salt is selected from the group consisting of methyl-tri-n-butyl ammonium salts, ethyl-tri-nbntyl ammonium salts, n-propyl-tri-n-butyl ammonium salts, tetra-n-butyl ammonium salts and n-amyl-tri-n-butyl ammonium salts and hydroxides derived therefrom.

4. Method according to claim 1 wherein the aromatic hydrocarbon is selected from the group consisting of naphthalene and alkyl derivatives thereof having 1 to 2 carbon atoms.

5. Method according to claim 1 wherein the aromatic hydrocarbon used is in an amount from 0.3 to 20 parts by weight per 100 parts by weight of the aqueous solution of one or more quaternary ammonium salts.

6. Method according to claim 1 wherein the concentra tion of the aqueous solution of one or more quaternary ammonium salts is from 10 to 50% by weight.

7. Method according to claim 1 including the further steps of dividing the resulting electrolyte into two layers and subsequently recovering the resulting dihydro aromatic hydrocarbon.

References Cited UNITED STATES PATENTS 2,477,580 8/1949' Condit 20473 R 3,485,726 12/ 1969' Misono et al 20473 R 3,542,656 11/1970 Suter et al 20473 R FREDERICK C. EDMUNDSON, Primary Examiner 

